Loop transmission facilities differ from communications bus transmission facilities in that the data stream in a loop network is sequentially receivable at each port disposed around the loop network whereas in a communications bus facility, the data is concurrently broadcast to all ports and the transit time is assumed to be negligible.
A number of loop transmission systems have been proposed including the well-known Pierce loop described in U.S. Pat. No. 3,731,002 and the Farmer-Newhall loop described in U.S. Pat. No. 3,597,549.
The loop networks have primarily been considered appropriate for the transmission of information in the form of digitally encoded data signals. The use of the loop network architecture for telephonic voice communication, however, has been hindered by the fundamentally different criteria applicable to different data and voice disciplines. Mainly, telephonic voice communication usually involves a dedication of a channel, or time slot, to a particular call with relatively long holding time compared to the time slot set-up requesting, addressing, acknowledgment, and reservation times. Certain kinds of data transmission on the other hand are characterized by being "bursty" and of short duration compared to the time slot set-up time. Moreover, certain kinds of data can tolerate a certain margin of delay in transmission and, if transmission is not successful, can withstand the need for a certain number of retransmission attempts.
A prior art system is described in the October/November 1980 issue of Computer Networks in an article by G. T. Hopkin entitled "Multimode Communications on the MITRENET"; that article describes voice and data communications in a bus system. A specific number of time slots in that communication bus are reserved for voice communications; the balance are reserved exclusively for data services. In the Mitrenet System, two channels, or time slots, must be reserved for voice communication between each pair of ports, one channel for voice transmission from a port A to a port B and a second channel for voice transmission from port B to port A. It is not possible to achieve concurrent data transmissions in one time slot among stations. The Mitrenet System having definite time slots for voice transmission may at any time be exposed to the need to meet data traffic while all of the available channels are in use. Likewise, such a system may be exposed to the need to provide actual channels to meet voice traffic when the remaining channels are not actually in use for data traffic. In neither of these situations, however, is there a definite channel assignment system available to seize additional channel capacity to handle the offered traffic.
A problem in such prior art is that complicated and inefficient procedures are required for requesting and reserving available time slots for transmitting either voice or data. Such procedures are particularly cumbersome and time consuming for the transmission of "bursty" data packets which oftentimes are shorter in duration than the time required for requesting, seizing, acknowledging and reserving a channel for the intended data transmission.